There are a lot of tasks in OOo that only require JPEGs in preview quality.
Currently previews in OOo are done by loading the entire image and then scaling
it down by using selected pixels of the full image. By using only the
low-frequency coefficients of the JPEG block's DCT there would be these benefits:
- less memory usage
  - by not needing the memory for the full blown images
- increased speed by
  - not needing complete IDCT
  - avoiding color transforms on pixels that will get ignored later
  - less work for Image->UniqueHash operations
- better preview quality
  - because the image is not disturbed by aliasing issues between high-frequency
    DCT coefficients and the statistical downsampling